Increase Of Deformation Strain Research Articles

Evolution of micropores and second phases during hot deformation of 2050 Al-Li alloy

Microporosity and second phases are typical components for casting 2050 alloy. In this paper, the evolution of both microporosity and the second phases during hot deformation of casting 2050 Al-Li alloy was studied. Firstly, based on industrial CT technology, the typical morphology characteristics of micropores during the hot deformation process of the alloy were studied and related statistics were carried out. The results show that with the increase of deformation strain, the volume, volume ratio, and porosity of the micropores gradually decreased, and the roundness gradually increased. Secondly, the detailed evolution mechanism of the second phases during hot deformation was studied by SEM, XRD, and TEM. It was found that with the increase of deformation strain, the thin plate T1 phase gradually undergoes shearing and exhibits bend-shaped morphology in the SEM image, which was essentially a process of stress transfer from Al matrix to the T1 phase. Meanwhile, the T1 phase also experienced thinning and secondary precipitation. This is mainly attributed to the intensive interaction between the alloying elements within T1 phase and the dislocations pile-up around T1 phase during the hot deformation process. While for the large-size intermetallic phase AlCuFeMn, it breaks up gradually and oxide inclusion is generated at the broken site, which provides favorable conditions for the initiation and propagation of cracks during hot deformation. From TEM observation, it is found that there is Al6Mn intermetallic phase in the alloy matrix with different morphologies under different deformation strains. When the deformation strain reaches 70%, a large number of nanometer Al6Mn particles are dynamically precipitated near the dislocations.

Deformation-induced dynamic precipitation of 14H-LPSO structure and its effect on dynamic recrystallization in hot-extruded Mg-Y-Zn alloys

Understanding dynamic precipitation in thermo-mechanical processing can shed light on the manufacturing of high-performance Mg-based alloys. This study investigates the dynamic precipitation of long-period stacking order (LPSO) phase with 14H structure and its effect on dynamic recrystallization (DRX) in the hot-extruded Mg98.5Y1Zn0.5, Mg97Y2Zn1 and Mg94Y4Zn2 (at. %) alloys. First, the simultaneous increase of Y and Zn atoms not only promotes the formation of interdendritic block 18R-LPSO phases during solidification but also facilitates the dynamic precipitation of intragranular lamellar and thin 14H-LPSO phases in coarse non-DRX and fine DRX grains, respectively, during hot extrusion. Second, the increased block 18R volume fraction and lamellar 14H number density significantly delays DRX. However, as the deformation strain increases, broken 18R fragments accelerate the DRX particle-stimulated nucleation (PSN) mechanism and fluctuant α-Mg/18R interfaces initiate the discontinuous DRX mechanism. In particular, the deformation kinking and dissolving of lamellar 14H activate the continuous DRX mechanism. Finally, the mechanical strength increases but ductility decreases with increasing the content of Y and Zn atoms for the hot-extruded Mg-Y-Zn alloys. The high strength of the hot-extruded Mg94Y4Zn2 alloy is attributed to fine DRX grain size, strong un-DRX grains, major block 18R and lamellar/thin 14H phases, and deformation kinking, while the high ductility of the hot-extruded Mg98.5Y1Zn0.5 alloy is responsible for large DRX volume fraction, high activity of deformation twins, and easy mobility of basal slip.

Effects of surface roughness on interfacial dynamic recrystallization and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo alloy joints produced by hot-compression bonding

The influence of surface roughness on the interfacial dynamic recrystallization kinetics and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo hot-compression bonding joints was systematically investigated. It is found that for the bonding interface of rough surfaces, elongated fine grains are formed at the bonding interface due to shear deformation of the interfacial area. As the surface roughness increases, the proportion of elongated grains drastically decreases as they further reorient to form equiaxed grains along the bonding interface of rougher surfaces resulting from severe incompatible deformation of the interface area. Meanwhile, high-density geometrically necessary dislocations accumulate around the interfacial recrystallization area to accommodate the incompatible strain and lattice rotation. A rotational dynamic recrystallization mechanism is thereby proposed to rationalize the formation of fine interfacial recrystallization grains during bonding of rough surfaces. In contrast to that of rough surfaces, bonding interface of polished surfaces exists in the form of straight interface grain boundaries without fine grains under the same deformation conditions. While with the increase of deformation strain, small grain nuclei form along the bonding interface, which is associated with discontinuous dynamic recrystallization assisted by strain-induced boundary migration of interface grain boundaries. Moreover, the bonding joints of rough surfaces show lower elongation compared with that of polished surfaces. This is because the formation of heterogeneous fine grains with low Schmid factor along the bonding interface of rough surfaces, leading to worse compatible deformation capability and thereby poor ductility of bonding joints.

Tensile twin evolution of Mg–3Al–1Zn magnesium alloy during high-strain rate deformation

In order to investigate the evolution of tensile twin in Mg–3Al–1Zn (AZ31) magnesium alloy under high-strain rate impaction, the cylindrical specimens were compressed along the rolling direction under the strain rate of 1600 s−1 by use of split Hopkinson pressure bars equipped with stop rings with different thicknesses. The microstructure was observed by electron back-scatter diffraction. The results show that tensile twinning dominates the early plastic deformation and the area fraction of tensile twins is increased with the increase of deformation strain. Furthermore, when the deformation strain is beyond ∼10%, plastic deformation is mainly dominated by dislocation slips. In addition, most of tensile twins activated in a single grain belong to a variant or a variant pair, which has the highest Schmid factor among all the variants.

Study on microstructure evolution of Ti–6Al–3Nb–2Zr–1Mo alloy with bimodal structure during hot compression in the near β region

Hot compression tests were performed different reductions on Ti–6Al–3Nb–2Zr–1Mo alloy with bimodal structure in the near β phase region to investigate the microstructure evolution. The stress–strain curves showed obvious flow softening. With the increase of deformation strain, the alloy gradually undergoes dynamic recrystallization (DRX). Simultaneously, the thickness of lamellar αs grains significantly decreases. The high-density dislocation area provides a suitable place for β grains nucleation. Subsequently, β grains grow in lamellar α grains and become approximately 60° with the lamellar α grains under the driving force of the dislocation. The calculated DRX critical true strain of the alloy deformed at 0.01 s−1 and 980 °C is 0.095. Discontinuous dynamic recrystallization (DDRX) is the main DRX mechanism, while continuous dynamic recrystallization (CDRX) is the assistant DRX mechanism. In addition, the twin boundaries provides opportune locations for the nucleation of DRX grains, thereby promoting the DRX of the alloy. Moreover, the initial texture disappears, and different new texture components appear during hot deformation. The texture density first increases with increased deformation strain. With the further increase of deformation strain, the texture density gradually decreases, due to the combined action of sufficient DRX and dynamic phase transformation.

On the microstructure evolution and nanocrystalline formation of pearlitic wheel material in a rolling-sliding contact

The objective of this study is to characterize the microstructural evolution of pearlitic wheel material in a rolling-sliding contact. Continuous observations were carried out from the top layer of a rolling-sliding contact surface to the undeformed substrate using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that the microstructure of the pearlitic wheel material after rolling-sliding contact exhibits an obvious gradient structure in the depth direction. The microstructure evolution process in the wheel material is as follows: during the initial stage of rolling-sliding contact, the pearlite undergoes plastic deformation, resulting in a decrease in the pearlitic lamellar spacing, a large amount of dislocations in the ferrite and a small amount of cementite fractures. The dislocations accumulate, tangle and form dense dislocation walls (DDWs) inside the ferrite as the deformation strain increases. The cementite begins to dissolve into particles and the dislocation tangles (DTs), and DDWs transform into sub-boundaries with small misorientations. The low-angle grain boundaries (LAGBs) transform into high-angle grain boundaries (HAGBs). The refined grains repeatedly undergo deformation, the dislocation cells form and the LAGBs transform to HAGBs, forming a nanocrystalline grain structure.

Crystallography and asymmetry of tensile and compressive stress-induced martensitic transformation in metastable β titanium alloy Ti–7Mo–3Nb–3Cr–3Al

The early stage (strain ranges from 3% to 7%) microstructure evolution of a newly developed metastable β titanium alloy Ti–7Mo–3Nb–3Cr–3Al under both tensile and compressive deformation at room temperature was investigated contrastively. Although the true strain-true stress curves and the corresponding strain hardening rate (SHR) curves of the samples under tensile/compressive deformation seems to distinct from each other, the dominate deformation product in all the processes proves to be Stress-Induced Martensite (SIM) with an orthorhombic structure. Strong variant selection of SIM happens under both tensile and compressive deformation. And lattice strain formed during β→α’’ phase transformation plays a key role to determine the variant selection process. In the 3% deformed samples (both tensile and compressive samples), the lath-shaped SIM with or without twinned structure is the main deformation product. With the increase of deformation strain, lath-shaped SIM remains to be the dominate deformation induced feature in the compressive samples, while the SIM configuration changes dramatically in the tensile samples. Morphology of SIM varies from grain to grain in the samples with 7% tensile strain, and the ratio of grains which have zigzag SIM with a 30° included angle increases obviously. Zigzag SIM with an approximately 150° included angle was found in the 7% compressive sample in the region near β grain boundaries. The features of these two kind of zigzag SIM were characterized in detail with the help of electron backscatter diffraction (EBSD) technology. Concrete analyses were conducted by employing the phenomenological theory of martensite crystallography (PTMC).

Effects of pre-stretching on microstructure and properties of WE43A-alloy after aging treatment

ABSTRACTEffects of pre-stretching on microstructure and mechanical properties of WE43A alloy were investigated by optical microscope, transmission electron microscope, scanning electron microscopy and electronic universal material testing machine. The experimental results showed that a large number of dislocations and twins were formed in the matrix with the increase of deformation strain. During subsequent aging, β1 phases precipitated in dislocation area and distributed in matrix in a combined manner, honeycomb network and random. The precipitation of β1 increased the ultimate tensile strength and yield strength of WE43A alloy to 272 and 241 MPa, in comparison with the samples without pre-stretching (226 and 180 MPa). The fracture morphology of sample with strain 9% was transgranular cleavage fracture.

Microstructural evolution and mechanical properties of an AgInCd alloy during compressive deformation process

The microstructure and mechanical properties of an AgInCd alloy during compressive deformation process have been investigated in present study. Results show that with increase of deformation strain, the deformation markings are formed in the deformed AgInCd alloys, the smooth and rectilinear annealing twin boundaries become bent and blurry. Lots of dislocations and dislocation tangles are accumulated in the deformed AgInCd alloys. Many long-length stacking faults are formed in the alloy with deformation strain 30%. With further increase of deformation strain, nanoscale deformation twins are widely formed in the AgInCd alloys. The microhardness of the AgInCd alloy increases and the standard deviation of microhardness inversely decreases due to the more uniform micro-deformation with deformation strain increasing. The uniform elongation firstly decreases and then increases as a result of the formation of nano-twins in the AgInCd alloy with deformation strain 50%.

Correlation of Deformation Strain with the Tensile Properties and Fracture Mode of the AA 2014 Alloy Using Simulations and Experiments

Abstract The influence of deformation strain on the tensile properties of the forged bar was studied in the present study. An aerospace structural Al alloy (AA 2014) was selected for the experiments. In order to understand the strain distribution inside the forged bar, the forging process was simulated using the DEFORM3D finite element software. The model was used to predict the stress and temperature distributions. Tensile sample locations were identified based on the deformation strain values predicted by the model. The samples were tested under both notched and unnotched conditions. Variation of tensile properties with the deformation strain was correlated with the microstructure. The fractography of the tensile samples was analyzed with the aid of scanning electron microscopy to identify the failure mode. The important results of the present study are: (1) the strength and ductility properties increase with the increase of deformation strain for both smooth and notched conditions. However, this increase was not correlated to the strain. This uncorrelated variation was attributed to the complex variability in the grain size/structure in the material. (2) Microstructure examination revealed that the variation in the grain size reduced with the increase of deformation strain. The observed grain refinement with the increase of deformation strain was mainly attributed to the strain induced grain fragmentation and dynamic recovery. (3) The fractography of the smooth specimens showed that ductile fracture features, such as dimples and ductile tear regions, were more significant above a strain of 1.34, indicating improved ductility. On the other hand, for the notched specimens, brittle fracture features such as cleavage facets and ridges were predominant for all the strain value conditions. (4) The fracture appeared to be transgranular type in both the smooth and the notched samples for the tested strain range.

Influence of hot working on microstructure and mechanical behavior of high nitrogen stainless steel

The relationship between microstructures and mechanical properties of a high nitrogen stainless (HNS) steel (0.65N–1.8Ni, wt%) manufactured by argon oxygen decarburization and continuous casting was investigated in this article. The plates with different thicknesses were obtained by thermo-mechanical control process. The results revealed that the plates prepared by hot rolled and solution treated possessed good balanced mechanical properties, i.e., satisfactory strength and higher toughness. Compared with hot-rolled and solution-treated plates, the hot-rolled plate had much higher tensile strength, but its impact toughness was extremely low. Furthermore, with the increase of deformation strain, the plate with finer grains, more precipitates and higher strength was achieved, but its plasticity decreased obviously. The worm-like carbides formed along the grain boundary during the finish rollings at 850 °C, which are detrimental to the toughness of hot-rolled plates. In addition, small amount of ferrite precipitated in the steel due to the non-equilibrium solidification during continuous casting, and to ensure full austenitic structure, composition design method was recommended.

Graded phase structure in the surface layer of NiTi alloy processed by surface severe plastic deformation

The graded phase structure and its evolution in the surface of NiTi alloy after surface mechanical attrition treatment are studied using transmission electron microscopy. The initial B2–B19′ phase transformation occurs at low strain level. As the deformation strain increases, the B19′ martensite phase transforms back to B2 phase giving rise to graded phases. Our study reveals that stress induced martensite transformation and deformation heating play important roles in the formation of this graded phase structure.

Constitutive relationship of IN690 superalloy by using uniaxial compression tests

The hot deformation behavior of IN690 superalloy was characterized in a temperature range of 1273–1473 K and a strain rate range of 0.01–10 s−1 using uniaxial compression tests on process annealed material. The constitutive relations between flow stress and effective strain, effective strain rate as well as deformation temperature were studied. It can be concluded that the flow stress significantly reduces with the deformation temperature of IN690 superalloy increasing. Whereas, there is a significant increase of flow stress when the strain rate increases from 0.1 s−1 to 10 s−1. Based on the hyperbolic-sine Arrhenius-type equation, a constitutive equation considering compensation of strain was developed. The activation energy and the material constants (Q, n and ln A) decrease as the deformation strain increases. The strain dependent term is successfully incorporated in the constitutive equation through a quartic equation. A good agreement between the experimental data and the predicted results has been achieved, indicating that the proposed constitutive equation and the methods of determing the material constants are suitable to model the high temperature deformation behavior of IN690 superalloy.

Interface structure and mobility in martensitic shape memory alloys

The interface structure and its mobility in the martensites of shape memory alloys (SMAs) are reviewed. The emphasis is placed on the evolution of interface structure during deformation and the relations of interface characteristics to the mobility in cold-deformed TiNi-based alloys and the types, structure and mobility of interfaces in Cu-based SMAs. With the increase of deformation strain in the cold-deformed TiNi alloys, the interface type of martensite variants changes and their mobility becomes bad gradually. For the Cu-based SMAs, the A/C and A/B interfaces have effective mobility and may move smoothly and become straight during motion. On the contrary, the mobility is not effective for the A/D interface.

Semisolid deformation of aluminium alloy 356 with non-dendritic structure

During semisolid deformation of aluminium alloy 356, as the deformation strain increases, the deformation stress first increases to reach a peak value, then decreases to a minimum value, after which the stress increases towards the end of the deformation. The deformation stress increases as the deformation rate increases at the beginning of the deformation. The increase in the stress towards the end of the deformation is larger as the deformation rate decreases. The difference between the first peak stress and the minimum stress increases as the deformation rate increases. The stress decreases as the deformation temperature increases. The difference between the first peak stress and the minimum stress decreases as the segregation rate increases. The increase in the stress towards the end of the deformation is larger as the segregation rate increases. The degree of segregation is the highest at the middle deformation temperature.

Semi-solid deformation of non-dendritic structures—I. Phenomenological behavior

Semi-solid deformation mechanisms are proposed to be four major mechanisms: liquid/flow (LF); flow of liquid incorporating solid particles (FLS); sliding between solid particles (SS); and plastic deformation of solid particles (PDS) mechanisms. The segregation phenomenon during semi-solid deformation, in which the solid particles segregate in the center portion, while the liquid segregates around the edge, is predicted by the phenomenological model. As the solid fraction and the deformation rate increase, the SS and PDS mechanisms become more dominant, as predicted by the phenomenological model. At small deformation strain, the center area dominated by the SS and PDS mechanisms is smaller for lower deformation rate than that for higher deformation rate; however, as the deformation strain increases, the situation is reversed, which is due to the composite deformation rate effects predicted by the phenomenological model. The effects of the deformation temperatures and deformation rates will be discussed in Part II, based on the present framework

Development of constitutive relationships for the hot deformation of boron microalloying TiAlCrV alloys

Isothermal compressive tests are performed on TiAlCrV and TiAlCrV+B alloys to study hot deformation behavior. A hyperbolic-sine Arrhenius-type equation is used to derive the constitutive equation to correlate the flow stress to the deformation temperature, and the strain rate to strain level. The results show that all experimental data of TiAlCrV and TiAlCrV+B alloys can be fully described using this hyperbolic-sine Arrhenius-type relationship. The activation energy, Q, the material constants, n and ln nA, of both alloys decrease as the deformation strain increases. The strain dependent term is successfully incorporated in the constitutive equation through the parameters, Q, n, and ln A. The preliminary results also suggest that the constitutive relationship between the flow stress and strain and between the strain rate and temperature can be used to predict the hot-workability map for the hot deformation of intermetallic alloys.